This invention is related to the field of wavelength division multiplexing. More specifically, it relates to using an automatic level control loop (ALC) in a dense wavelength division multiplexing (DWDM) system.
Wavelength division multiplexing (WDM) is a method of increasing the information capacity of a fiber optic system. Instead of using just one optical information source transmitting over one narrow spectral width, WDM takes advantage of the full transmission capability of an optical fiber by simultaneously transmitting signals from a plurality of optical information sources over different channels on the same optical fiber. The sources transmit information over different peak wavelengths which are spaced far enough apart so as to avoid interference between adjacent signal sources. Transmitting information in this manner utilizes the full bandwidth of the fiber.
Light generating sources are used as transmitters in optical systems (along with a modulator and an information source). A WDM system can use a plurality of fixed frequency light sources or one tunable laser source.
One way to tune lasers is by varying the wavelength or, equivalently, the laser frequency. At the same time, it is preferable to minimize other undesirable or unwanted effects. However, this is not always possible. For example, when tuning a currently available distributed feedback (DFB) laser, it may not be possible to vary the wavelength without causing other undesirable effects. In addition, with dense wavelength division multiplexing (DWDM) applications, optical frequency or wavelength stabilization of lasers may be needed. Etalons are an example of a stable optical filter used for wavelength stabilization.
Since the laser wavelength is a function of temperature, lasers can be tuned with temperature. Unfortunately, other parameters, such as optical power output, also change when tuning using temperature. An optical power control system may then be used to control the optical power. This generally does not present a major problem since this feature is found in many laser systems. For example, in U.S. Pat. No. 6,236,667 the wavelength control system feeds a control current or control voltage to a control terminal which changes the laser wavelength. A thermoelectric cooler (TEC) coupled to the wavelength control system receives this control signal and responds thereto by changing the temperature of the laser and thereby the wavelength. U.S. Pat. No. 6,236,667 is hereby incorporated by reference.
Typically, when transmitting data in a channel, unwanted or undesired frequencies are transmitted along with the desired signal. These signals take the form of sidelobes which are only 30 dB below the signal peak. These sidelobes can leak into other channels, thereby corrupting the signals in the other channels.
Generally, data channels work over a very narrow dynamic range and don""t require much power. Therefore, crosstalk from sidelobes is usually not a problem. However, a dense wavelength multiplexing (DWDM) system comprising an automatic level control (ALC) loop contains many channels. ALC loops require significantly more power to compensate for all the channels. This additional power results in even higher sidelobes (or sidelobes containing more power) than a typical data channel. As a result, the signals from these unwanted sidelobes leak into other channels.
An optical bandpass filter can be used to reduce the sidelobes. However, introduction of a bandpass filter can produce nonlinearities in an ALC loop. Wavelength lockers are currently used to control nonlinearities in ALC control loops. However, they are very complicated and only work over a narrow dB dynamic range. Consequently, wavelength lockers may not work within ALC control loops having large dB dynamic ranges.
The present invention is an automatic level control loop comprising an automatic level control circuit comprising a controller, a filter operably connected in feedback with the automatic level control circuit and a thermoelectric cooler loop operably connected to the automatic level control circuit.
In another preferred embodiment, the automatic level control circuit further comprises an amplifier operably connected between said photodiode and a second input of an error amplifier, an integrator connected in between the output of said first error amplifier and the laser diode, the laser driver connected in between the output of the error amplifier and the laser diode, and a first digital-to-analog converter operably connected between the input of the first error amplifier and the output of the controller.
In still another preferred embodiment, the thermoelectric cooler loop further comprises a thermoelectric cooler driver operably connected in series with the integrator, wherein the thermoelectric cooler driver and the integrator are operably connected between the output of the second error amplifier and the thermoelectric cooler, a second digital-to-analog converter operably connected between a first input of the second error amplifier and a second output of the controller; and an amplifier operably connected between the thermistor and the second input of the second error amplifier.
In still another preferred embodiment, the present invention is a method of maintaining an output power level using an automatic level control loop, comprising the steps of providing a desired power level by outputting a reference signal. Next, a portion of the a laser diode""s output power is fed back, filtered and converted to a feedback voltage. Next, the fed back portion of filtered output power is subtracted from the reference voltage, thereby producing a difference signal. This difference signal is used to drive the laser diode.
In still another preferred embodiment, the present invention is a method of stabilizing output frequency using a thermoelectric loop, comprising the steps of monitoring a laser diode""s output power and determining whether said laser diode""s output frequency has drifted. To tune the laser diode, the following steps are performed. A digital value is read from a calibration table, a desired output wavelength is provided by outputting a control signal, a temperature of a thermoelectric cooler is sensed and converted to a feedback signal. Next, the feedback signal is subtracted from the control signal producing a difference signal. This difference signal is used to drive a thermoelectric cooler, which in turn, changes a temperature of the laser diode.